For rewriteable data media, on which data can be appended to a partially recorded medium, and on which previously written data can be erased and overwritten, data formats commonly provide data gaps for accommodating angular velocity variations between drives, and for accommodating write clock drift. Rewriteable data formats also commonly provide clock synchronization patterns for adjusting the write clock frequency and phase. For example, magnetic discs and tapes are typically formatted into sectors, with each sector including a preamble for synchronizing a write clock, and with each sector including extra space at the end to allow for variations in media velocity. Synchronization patterns and data gaps reduce effective data capacity because they occupy space that could otherwise be occupied by user data.
In contrast, some proposed formats for rewriteable Digital Versatile Discs (DVD) do not have clock synchronization fields or extra space at the end of sectors. One rewriteable DVD format specifies a land and groove structure, with the grooves having a sinusoidal radial displacement (called wobble), and for the particular format, groove wobble is used to synchronize a write clock. In general, data is encoded in the timing of transitions between marks and spaces. The particular format specifies that certain marks must be written within a specified range of spatial positions relative to a spatial zero-crossing of the wobble. There is a need for writing data marks and spaces at precise positions, and to be able to verify the placement precision. In general, the beginning and end of data marks and spaces are defined by edges of a write clock. Accordingly, a necessary first step in controlling placement precision is an accurate synchronized write clock. However, there are various signal path delays that may vary with time and temperature, and signal path delays that may vary from drive to drive. In addition, the impact of these signal path delays may vary depending on the angular velocity at which the disc is written. There is a further need for an ability to control and verify spatial placement precision of data marks and spaces, even with variable unknown path delays.
As discussed above, some optical disc formats have a land and groove structure, with at least one sidewall of the groove having a sinusoidal radial displacement. Groove wobble may be frequency modulated to encode time or address information, or groove wobble may be used to synchronize a write clock. Some optical disc formats provide spatial features, such as notches in groove sidewalls, that are used for index marks, sector addresses, or for additional phase control of a write clock. See, for example, U.S. Pat. No. 5,933,411 (Inui et al.), and U.S. Pat. No. 5,852,599 (Fuji). See also, for example, M. Yoshida et al., “4.7 Gbyte Re-writable Disc System Based on DVD-R System”, IEEE Transactions on Consumer Electronics, Nov. 1, 1999, v 45, n 4, pp 1270-1276 (Yoshida et al.).
FIG. 1 (prior art) illustrates a representative example disc drive. In the following discussion of FIG. 1, it will be seen that an accurate clock is a necessary but insufficient condition for precise spatial placement of marks. One must also compensate for various signal path delays.
In many optical disc drives, a single optical detector is used to generate a data signal, a radial position error signal, a focus error signal, and perhaps a wobble signal. FIG. 1 illustrates various lumped path delays for an optical disc drive using one optical detector for multiple functions. In FIG. 1, a light spot 100 is focused onto a data layer of an optical disc. Light reflected from the disc passes through various optical components before being detected by an optical detector 104. In FIG. 1, optical path delays between the disc and the detector 104 are lumped as Delay 1 (102). As depicted in FIG. 1, the optical detector 104 is divided into four sections (A,B,C,D), with each section providing a separate signal. The sum of the four signals (A+B+C+D), with some electronic filtering and processing, is the analog Read Data signal (108). Read Data signal path delays, due to filtering and other electronic processing, are lumped as Delay 2 (106). The analog Read Data signal 108 is received by an analog comparator 130, and compared to a reference voltage. The binary output of the analog comparator is the binary Read Data signal 132.
A radial position error signal, called a Radial Push-Pull (RPP) signal, is derived by subtracting appropriate pairs of the quad detector signals, for example (A+D)−(B+C). For media with wobbled grooves, the wobble signal is a high frequency modulation of the relatively low frequency RPP signal. In FIG. 1, various electronic filtering and processing delays for the RPP/wobble signal are lumped as Delay 3 (110). If the wobble signal is used for synchronization of a write clock signal, the wobble signal is typically received by a Phase-Locked Loop (PLL, 112). The output of the PLL is used for a Write Clock (114). A Write Data signal (116) is synchronized to edges of the Write Clock (114), as controlled by a latch 118 to generate a Write Intensity signal (120). A Laser Intensity circuit 126 is controlled either by the Write Intensity signal (120) or by a Read Intensity signal, and the Laser Intensity circuit then controls the intensity of a laser diode light source. In FIG. 1, path delays in driving the Laser Intensity circuit, as well as any optical path delays are lumped as Delay 4 (128).
Typically, Delay 1 and Delay 4 are negligible. Delay 2 and Delay 3, however, are significant, and both may vary with time and temperature, and may vary from drive to drive. The relative effects of these delays also varies with the angular velocity of the disc. For example, if a disc is partially written in a drive at 1× angular velocity, and rewritten in a drive at 2× angular velocity, the delays have a different effect for the 2× drive relative to the 1× drive.
Consider the problem of writing a new mark at a precise spatial position relative to a spatial zero-crossing of wobble, or writing a new mark relative to an existing mark. One could detect a zero-crossing in a wobble signal, wait the proper number of Write Clock (114) cycles, and write the beginning of the new mark. Alternatively, one could detect the end of an existing mark using the Read Data signal (108), wait the proper number of Write Clock cycles, and write the beginning a new mark. Typically, wobble zero crossings or mark edges would be averaged over many transitions using a phase-locked loop. The proper number of Write Clock cycles may be known for calibrated drives, but may vary over time and may vary from drive to drive. The problem is that if Delay 2 (106), Delay 3 (110), and delay in the PLL 112 are unknown and variable, then there is uncertainty in the time at which a new mark should be written relative to a wobble signal, as sensed in the RPP signal, or relative to an edge of an existing mark, as sensed in the binary Read Data signal. As a result, there is some variation, in the spatial position of the new mark relative to spatial wobble, or in the spatial position of the new mark relative to the existing mark, or the new mark, that may be sufficient to cause a data error during reading. If a leading edge of a new mark is to be precisely spatially located relative to a spatial zero-crossing of wobble, or relative to the trailing edge of an existing mark, the system must compensate for Delay 2, and Delay 3, and the delays in the PLL 112 and the latch 118.
Consider, for example, Fuji (cited above) and Yoshida et al. (cited above). In Fuji, and in Yoshida et al., spatial features are used to synchronize a write clock. However, as discussed above, accurate write clock generation is necessary but not sufficient. The write clock is only part of the problem. An accurate clock enables relative precision, so that a mark may be written consistently at some latency after detecting a feature in the read signal or wobble signal, but the latency is unknown and may vary over time and from drive to drive. In proposed specifications for rewriteable DVD with a 4.7 Gbyte capacity per writing surface, an absolute spatial position accuracy is required. Specifically, in one proposed standard, certain specified marks must be spatially placed within ±5 channel bits of the spatial zero crossing of a spatial wobble having spatial period of 32 channel bits.
There is need for a capability to verify that marks have been spatially placed relative to a spatial wobble within a certain tolerance. There is a further need for a drive to be able to place marks at precise absolute spatial positions.